1. Field of the Invention
This invention is applicable to data transmission using a ring network, and more particularly it relates to a loop-back method for recovering a line from failure by loop-back in a ring line which transmits data in modes such as packets, ATM (asynchronous transmission mode) and STM (synchronous transmission mode).
2. Background of the Invention
FIGS. 25 and 26 are explanatory views to describe a prior art loop-back method used in a ring network. A ring network which connects four nodes with a dual ring line is exemplified herein for explanation.
The ring network has double ring lines, one of which is a counterclockwise ring used as a working line 111 and the other of which is a clockwise ring used as a protection line 112. Four nodes 113, 114, 115, 116 are arranged on the lines. Each of the nodes is provided with a node input terminal which is expressed by affixing i after its reference numeral, a node output terminal expressed by affixing o likewise to its reference numeral, an insertion into the ring line expressed by A (and by a white circle in the figure), and a branch in the ring line expressed by D (and by a black circle in the figure).
Description is made of restoring the line by reconfiguring using a loop-back when a transmission failure 150 takes place between the nodes 115 and 116 on the dual ring network of this structure. When a failure takes place, the node 113 on the ring (which is usually referred to as a ring monitor node) detects the failure and searches the nodes 115 and 116 between which the failure took place. Then, it recovers the line from failure by looping back the sides of the transmission line facing the failure between the two nodes 115 and 116 as shown in FIG. 26.
Such a technique, however, takes a very long time to accomplish because of the extremely complicated operations involved in the search for which terminals are the failing terminals on the lines 111 and 112. More specifically, the search usually starts from the monitor node (in this example, the node 113), and advances one farther node at a time by commanding loop-back at each adjacent node. It eventually determines the failed node as the one from which the loop-back signal has not returned and makes a loop-back for that node or one node before that node.
The operation sequence for searching the failed terminals will now be described.
The monitor node 113 connects the output thereof on the counterclockwise working ring line 111 with an input of the clockwise protection ring line 112. This makes the monitor node 113 a loop-back node. If it normally receives a signal from the protection line 112, it removes the loop-back, and then the monitor node 113 issues a loop-back command to the node 114 in order to extend the position at which loop-back occurs. When receiving the command, the node 114 connects the output thereof on the counterclockwise working ring line 111 with an input of the clockwise protection ring line 112. By this operation, the signal from the monitor node 113 is passed through the counterclockwise working ring line 111, looped back at the node 114 inserted into the clockwise protection ring line 112, and returned normally to the node 113. When the monitor node 113 receives the signal normally from the protection ring line 112, it cancels the loop-back command for the node 114. A similar operation is repeated for the node 115 by extending the loop-back node farther from the monitor node 113, and if the monitor node 113 receives a signal normally, it commands the node 116 into a loop-back mode. But since a failure 150 took place between the nodes 115 and 116, the signal will not be received normally from the line 112. This is either because the node 116 cannot receive the loop-back command or because the normal signal cannot be returned due to the failure 150 on the transmission line although the node 116 does configure as a loopback. In either case, since the normal signal is not returned, the node 113 sends a command to cancel the loop-back to the node 116, and sends a command for loop-back to the node 115 which is one node before the node 116. This makes the node 115 a loop-back node (LB1).
The technique repeats similar loop-back operations in the opposite direction (provided that the line on the output side of the node 113 is referred to as a protection ring line 112 and that on the receiver side as the working ring line 111). The first command configures the next node 116 as a loopback node so that the node 116 connects the clockwise line 112 with an input of the counterclockwise ring line 111 and the node 116 becomes a loop-back node (LB2). As shown in FIG. 26, this recovers the lines from the failure 150 by connecting the counterclockwise working ring line 111 which has been cut off between the nodes 115 and 116 due to the failure with the clockwise protection ring line 112 by means of the two loopback nodes 115 and 116.
As described in the foregoing, the loop-back operation in the prior art is highly complicated, involving a large number of difficult steps and therefore taking a lot of time for recovery from a failure.
This invention aims to solve the aforementioned defects encountered in the prior art and to provide a loopback method in a dual ring line which simplifies the recovery operation and requires less time.